The first -40º temperature of the winter in Alaska

November 24th, 2018 |

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

A sequence of Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images centered over the North Slope of Alaska (above) showed a few patches of thin stratus cloud drifting westward on 24 November 2018. Ample illumination from the Moon — which was in the Waning Gibbous phase, at 98% of Full — maximized the “visible image at night” capability of the Day/Night Band. A faster animation of Infrared images helped to emphasize the westward motion of multi-year drift ice in the Beaufort Sea as it collided with the growing wedge of first-year land-fast ice off the northeast coast of Alaska.

In areas with deeper snow cover that remained generally cloud-free for long periods of time, temperatures at first-order stations dropped into the -20s and -30s F; a low of -35ºF was recorded at Nuiqsut (PAQT). A closer look at the 2314 UTC Infrared image (below) revealed surface brightness temperatures as cold as -47ºC or -53ºF (lighter shades of yellow) in the valleys near Galbraith Lake (PAGB).

Suomi NPP VIIRS Infrared Window (11.45 µm) image at 2314 UTC [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) image at 2314 UTC [click to enlarge]

The RAWS site at Umiat Airfield (PAUM) registered a minimum temperature of -40ºF (hourly summary) at 2123 UTC on 24 November (below) — this was the first reliable -40º temperature of the 2018/2019 Winter season in Alaska. Farther to the east, the HADS site at Sagavanirktok recorded a low of -44F, but that max/min temperature data was flagged as being suspect (red) by Mesowest quality control.

Minimum and maximum temperatures for the 24-hour period ending at 20 UTC on 25 November [click to enlarge]

Minimum and maximum temperatures for the 24-hour period ending at 20 UTC on 25 November [click to enlarge]

Banner cloud in Alaska

November 7th, 2018 |

Topography + Suomi NPP VIIRS Infrared Window (11.45 µm) images, with/without overlays of NAM12 250 hPa winds [click to play animation | MP4]

Topography + Suomi NPP VIIRS Infrared Window (11.45 µm) images, with/without overlays of NAM12 250 hPa winds [click to play animation | MP4]

Suomi NPP VIIRS Infrared Window (11.45 µm) images (above) showed a well-defined banner cloud extending from the Brooks Range in northern Alaska to the Beaufort Sea on 07 November 2018. Overlays of NAM12 model 250 hPa winds revealed the presence of a branch of the polar jet stream flowing northeastward over the region. Strong southwesterly winds interacting with the topography of the Brooks Range created a standing wave which led to the formation of the banner cloud.

In a comparison of Suomi NPP VIIRS Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images (below), note the significantly warmer 3.74 µm cloud-top brightness temperatures — as much as 40 to 50ºC warmer at 2009 UTC when the sun angle was highest over Alaska — caused by enhanced solar reflectance off the very small ice crystals at the top of the banner cloud.

Suomi NPP VIIRS Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images [click to play animation | MP4]

Suomi NPP VIIRS Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images [click to play animation | MP4]

GOES-15 (GOES-West) Water Vapor (6.5 µm) and Infrared Window (10.7 µm) images (below) showed that a large banner cloud had persisted downwind of the Brooks Range fpr much of the day.

GOES-15 Water Vapor (6.5 µm, top) and Infrared Window (10.7 µm, bottom) images [click to play animation | MP4]

GOES-15 Water Vapor (6.5 µm, top) and Infrared Window (10.7 µm, bottom) images [click to play animation | MP4]

Alaska: a thunderstorm, single digits and a volcano

September 25th, 2018 |

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images (above) captured an unusually late thunderstorm that produced small hail at Anchorage PANC (surface observations) on 24 September 2018. The coldest cloud-top infrared brightness temperature was -53.8ºC, which was colder than the -46.3ºC tropopause temperature on the 00 UTC Anchorage sounding. This particular thunderstorm (Anchorage averages only 1-2 per year) even featured a wall cloud:



In far northeastern Alaska, snow cover across the North Slope and Brooks Range was evident in a sequence of Suomi NPP VIIRS Visible (0.64 µm) images (below). Since there were also areas of low cloud present (both north and south of the primary snow cover), the VIIRS Shortwave Infrared (3.74 µm) images could be used to discriminate between these low clouds — whose supercooled water droplets were effective reflectors of solar radiation, making then appear warmer or darker gray — and the cloud-free areas of snow cover.

Sequence of 4 Suomi NPP VIIRS Visible (0.64 µm) and Shortwave Infrared (3.74 µm) images [click to enlarge]

Sequence of 4 Suomi NPP VIIRS Visible (0.64 µm) and Shortwave Infrared (3.74 µm) images [click to enlarge]

The presence of this snow cover aided strong nighttime radiational cooling as a ridge of high pressure moved over the North Slope (surface analyses), and on the following morning temperatures dropped as low as 4ºF (the temperature later reached 3ºF at Toolik Lake):

Finally, along the Alaska Peninsula, Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images revealed the bright glow and hot thermal signature of the ongoing eruption of Mount Veniaminof at 1204 UTC and 1344 UTC (below).

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1204 UTC [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1204 UTC [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1344 UTC [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1344 UTC [click to enlarge]

Coincidentally, on this day GOES-17 began a test of Mode 6 operation which performs a Full Disk scan every 10 minutes. Although the Alaska Peninsula was on the extreme northwest limb of the Full Disk scan, Veniaminof’s thermal anomaly or “hot spot” (darker black pixels) could still be detected and monitored at 10 minute intervals using Shortwave Infrared (3.9 µm) imagery (below). However, an increase in layered cloud cover southeast of that area later in the day (in tandem with the extreme satellite view angle) eventually masked the warm thermal signature — a more direct view from overhead with Suomi NPP VIIRS still showed a very hot volcano summit (96.9ºC) at 2156 UTC.

GOES-17 Shortwave Infrared (3.74 µm) images [click to play animation | MP4]

GOES-17 Shortwave Infrared (3.74 µm) images [click to play animation | MP4]

Since there were no significant ash emissions from Mount Veniaminof on this day, no volcanic signature was evident on GOES-17 “Red” Visible (0.64 µm) imagery (below).

GOES-17

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

* GOES-17 images shown here are preliminary and non-operational *

Snow cover in the Brooks Range and North Slope of Alaska

September 2nd, 2018 |

Suomi NPP VIIRS Infrared Window (11.45 µm) images on 01 and 02 September [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) images on 01 and 02 September [click to enlarge]

A low moved eastward across the Beaufort Sea on 01 September 2018, bringing a cold front southward across the North Slope and Brooks Range in far northern Alaska (surface analyses). A sequence of Suomi NPP VIIRS Infrared Window (11.45 µm) images (above) showed the clearing of high/cold clouds in the wake of the frontal passage.

The upslope flow of cold air helped to generate accumulating snowfall across that region — prompting a Winter Storm Warning to be issued for the eastern Brooks Range, where 4-8 inches was expected at higher elevations — and some of the resulting snow cover was seen on a Suomi NPP VIIRS Day/Night Band (0.7 µm) image at 1415 UTC or 6:15 am local time on 02 September (below). A comparison with the corresponding VIIRS Infrared Window (11.45 µm) image and Topography is also shown. The darker shades of brown on the topography image correspond to elevations of 6000-8000 feet in the Brooks Range.

Suomi NPP VIIRS Day/Night Band (0.7 µm), Infrared Window (11.45 µm) and Topography images [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm), Infrared Window (11.45 µm) and Topography images [click to enlarge]

Later in the day on 02 September, additional clearing of patchy low clouds revealed more of the snow cover, as seen in a toggle between VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Topography images (below). Supercooled water cloud droplets are efficient reflectors of incoming solar radiation, making patches of low cloud appear darker shades of gray on the Shortwave Infrared image (helping to identify low clouds over snow cover).

Suomi NPP VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Topography images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Topography images [click to enlarge]

At 2124 UTC (or 1:24 pm local time), a 30-meter resolution Landsat-8 False Color Red-Green-Blue (RGB) image viewed using RealEarth (below) provided a more detailed view of a portion of the snow cover. Snow and ice appear as shades of cyan in this type of RGB image — which is created by combining Landsat bands 6 (1.61 µm), 5 (0.865 µm), and 4 (0.655 µm) as Red, Green, and Blue — and numerous small ice floes can also be seen off the coast.

Landsat-8 False Color RGB image [click to enlarge]

Landsat-8 False Color RGB image [click to enlarge]

On a side note, farther to the west an interesting pattern of contrails was seen in VIIRS Visible and Infrared Window images at 2046 UTC (below). On the Visible image, note that the darker contrail shadows cast onto the surface are displaced about 15 miles to the north (due to the low sun angle); the contrail features exhibited Infrared brightness temperatures of -10 to -15ºC. These contrail patterns were generated by military aircraft performing training exercises: similar features have been noted over California and North Dakota.

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images [click to enlarge]

A curved portion of one of these contrails was seen on web camera images looking south from Atqasuk (below).